The present invention relates to a power-assisted steering system in which a steering mechanism of a vehicle is provided with an auxiliary steering force by driving an electric motor.
Conventionally, an electric power-assisted steering system has been in use in which a steering mechanism is provided with an auxiliary steering force by driving an electric motor in accordance with a steering torque exerted on a steering wheel by a driver of the vehicle. A torque sensor is provided in the electric power-assisted steering system for detecting a steering torque exerted on the steering wheel, which is a manipulating device for steering the vehicle, and a target value for a current (hereinafter, referred to as a “target current value”) that is to be supplied to the electric motor is set based on the steering torque detected by the torque sensor. Then, a command value that is to be provided to a driving device for the electric motor is generated through a proportional integration operation based on the steering torque detected by the torque sensor. The electric motor driving device includes a PWM signal generating circuit for generating a pulse width modulation signal (hereinafter, referred to as a “PWM signal”) having a duty ratio which corresponds to the command value and a motor driving circuit which is configured by utilizing a power transistor which is on and off in accordance with the duty ratio of the PWM signal and applies to the electric motor a voltage which corresponds to the duty ratio. A current caused to flow to the electric motor by the application of the voltage is detected by a current detector, and a difference between the target current value and the detected current value so detected is used as a deviation for generating the command value.
In recent years, there have been cases where a brushless motor is used in an electric power-assisted steering system like one described above. It is a general practice with the electric power-assisted steering system in which a brushless motor having three phases is used that there are provided current detectors for detecting current flowing to any two of the three phases of the brushless motor, for example, current flowing to, for example, the U-phase and V-phase thereof. Note that current then flowing to the W-phase can be obtained through a calculation.
In a configuration like one described above, there exists an intrinsic error resulting from a difference caused intrinsically between the individual current detectors in a gain which is a ratio of an output value of the current detector to a measured current value (an input value) thereof. Note that this gain corresponds to the inclination of a straight line indicating a corresponding relationship between the measured value and the output value. In case a motor control is implemented without considering the influence of the gain difference of the current detector, there is caused a torque ripple, and the driver feels a feeling of physical disorder when manipulating the steering wheel.
With a view to eliminating the influence from the gain difference of the current detector that has been described above, there exists a conventional electric power-assisted steering system in which an output value of a current detector is regulated by means of a hardware (refer to, for example, a patent literature No. 1). In addition, there exists an electric power-assisted steering system in which a reference current detector which is accurately regulated at the time of production is provided outside the electric power-assisted steering system, whereby an output value of the current detector incorporated within the system is corrected or regulated by means of a predetermined software by comparing an output value of the reference current detector with the output value of the current detector so incorporated.
Patent Literature No. 1: JP-A-10-75598
With the conventional configuration in which the output value of the current detector is regulated by the hardware, however, the production costs are increased due to the additional part, and in the event that a regulating process of the hardware has to be added, man-hours also have to be increased.
In addition, in the event that the output value of the incorporated current detector is corrected or regulated by means of the software based on the reference current detector provided outside the system, the reference current detector which is accurately regulated has to be prepared. Furthermore, since it cannot be avoided that a predetermined error is generated when regulating the output value of the current detector, it is normal that the output value of the reference current detector does not coincide with the corrected or regulated output value of each current detector completely. Here, even with a deviation between the output value of the reference current detector and the output value of each current detector being relatively small, when output values of a plurality of current detectors are compared with each other, there may be a case where the (relative) deviation becomes relatively large. As this occurs, the aforesaid torque ripple may take place, causing thereby the driver to feel a feeling of physical disorder when turning the steering wheel.